Tim
As you refer to 'sniper' upgrade for 50 points (and you come from the Antipodes), I presume you are using "the Handbook". Giving all four scout marines 'sniper' is both more playable and more sensible that the +25 to upgrade one stand, and IIRC was universally accepted (well, all but JJ anyway
). I would agree that it is just about worth +50 as you should get 1-2 hits to be allocated as you desire.
Adding a single Dreadnought to the Tacticals is a bit of a mixed blessing because it both reduces the formations strategic deployment options (you cannot really march on if needed) but worse, it allows AT fire to target the formation. IMHO it is not really effective; you have robbed the Tacs of their transport but not really provided an effective alternative.
IMHO a better option is to put 2x Dreadnoughts with Devastators which still get the same deployment options as the Tacticals while improving their AT firepower; or to add 2x Dreadnoughts to Terminators to boost their CC assault with similar deployment options. Note they also provide cover in both cases.